Radiotelephone systems are commonly employed to provide voice and data communications to a plurality of mobile units or subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
As illustrated in FIG. 1, a cellular radiotelephone communication system 20 as in the prior art includes one or more mobile stations or units 21, one or more base stations 23 and a mobile telephone switching office (MTSO) 25. Although only three cells 36 are shown in FIG. 1, a typical cellular network may comprise hundreds of base stations, thousands of radiotelephones and more than one MTSO. Each cell will have allocated to it one or more dedicated control channels and one or more voice channels. Through the cellular network 20, a duplex radio communication link 32 may be effected between two mobile stations 21 or, between a radiotelephone 21 and a landline telephone user 33. The function of the base station 23 is commonly to handle the radio communication with the mobile station 21. In this capacity, the base station 23 functions chiefly as a relay station for data and voice signals.
A typical base station 23 as in the prior art is schematically illustrated in FIG. 2. The base station 23 includes a control unit 34 and an antenna tower 35. The control unit 34 includes the radio control group 37, or RCG, and an exchange radio interface (ERI) 38. The ERI 38 receives data from the RCG 37 and transfers it to the MTSO 25 on a dedicated link 45, and receives data from the MTSO 25 and sends it the RCG 37 for subsequent transmission to a radiotelephone 21. The radio control group 37 includes electronic equipment necessary to effect radio communications, including one or more transmitters 54 and receivers 55. The transmitters 54 and the receivers 55 transmit and receive radiotelephone communications signals 70 via antennas 63, 65, which are desirably mounted at some distance above the ground.
A typical radiotelephone 21 as in the prior art is illustrated in FIG. 3. The radiotelephone 21 typically includes many of same kind of communications components found at a base station 23, such as a transmitter 54 and receiver 55. Typically, the transmitter 54 and receiver 55 share a common transmit/receive antenna 64, which may be mounted on the radiotelephone 21 or on other structures such as vehicles. Typically, the antenna 64 is an omni-directional antenna which, although having generally low gain in comparison to directional antennas, allows the user to operate the radiotelephone 21 at convenient orientations. Other components of a typical radiotelephone 21, such as a controller, keypad, display, microphone and speaker are not shown in FIG. 3.
As illustrated in FIG. 4, satellites may be employed to perform similar functions to those performed by base stations in a conventional terrestrial radiotelephone system, for example, in areas where population is sparsely distributed over large areas or where rugged topography tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 100 typically includes one or more satellites 110 which serve as relays or transponders between one or more earth stations 130 and radiotelephones 120. The satellite communicates radiotelephone communications over duplex links 170, 180 to radiotelephones 21 and the earth station 130. The earth station may in turn be connected to a public switched telephone network 30, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple ninimally-overlapping beams 150, each serving distinct geographical coverage areas 160 in the system's service region. Thus, a cellular architecture similar to that used in terrestrial cellular radiotelephone systems may be provided.
Traditional analog radiotelephone systems typically employ frequency division multiple access (FDMA), wherein a radiotelephone communications channel includes one or more carrier frequency bands which are dedicated to a user for the duration of a particular session or call. To provide greater channel capacity and to more efficiently use the radio spectrum, however, many conventional radiotelephone systems operate using time division multiple access (TDMA) or code division multiple access (CDMA).
Communications in a TDMA system occur over a series of sequential time slots on each carrier frequency band, with individual users allocated one or more time slots on a band. Each user communicates with the base station using bursts of digital data transmitted during the user's assigned time slots. Instead of assigning time slots to each user, CDMA systems assign users one or more unique spreading codes, each of which corresponds to a coded modulation sequence used in transmitting radiotelephone communications signals. The coded modulation "spreads" the transmitted signals in the frequency domain, and a receiving station uses the same code to "despread" the coded signals and recover the transmitted communications. Thus, in a typical TDMA system, a radiotelephone communications channel includes one or more time slots allocated on one or more carrier frequency bands, while in a typical CDMA system, a channel includes a spreading code/carrier frequency combination.
As illustrated in FIG. 5, a transmitting station 500 typically includes a transmit processor 510 which processes radiotelephone communications 505 to produce a communications symbol sequence 515 corresponding to the radiotelephone communications 505. This sequence is typically is communicated over a radiotelephone communications channel 520, producing a sequence of communications symbol data 525 at a receiving station 501. The receiving station 501 typically includes a receive processor 530 which processes the data sequence 525 produced by the radiotelephone communications channel 520 to recover radiotelephone communications 635. Those skilled in the art will understand that transmitting station 500 and receiving station 501 may be base stations, radiotelephones, satellites, earth stations or similar radiotelephone communications stations. Those skilled in the art will also appreciate that the radiotelephone communications channel 520 has an associated transfer characteristic 522 which represents a relationship between an input to the channel 520 and an output from the channel 520, typically expressed as a complex gain having magnitude and phase components.
Elements of the radiotelephone communications channel 520 affect the transfer characteristic 522. These elements include the signal transmission medium 72, i.e., the atmospheric signal path across which radiotelephone communications signals 70 are transmitted, which may introduce fading and interference into the radiotelephone communications signals 70. The fading may include long-term fading due to variations in terrain along the signal propagation path, as well as short-term multipath fading due to reflections from features such as buildings which cause fluctuations in received signal strength and other distortions at a receiving station. Mobile terrestrial radiotelephone communications are particularly susceptible to short-term multipath fading because the signal pathways tend to be close to the ground. Satellite-to-ground communications may also be susceptible to interference from surrounding man-made and natural features, as well as other types of signal losses, such as those caused by the Doppler shift associated with signals communicated to and from a communications satellite moving at a high speed with respect to the radiotelephones it serves. Other elements of the channel 520 which may affect the transfer characteristic 522 include transmitting and receiving components commonly found in base stations, satellites, earth stations, radiotelephones and the like, such as transmitters 54, transmit antennas 63, receive antennas 65, receivers 55.
Various techniques are conventionally employed to deal with the propagation losses, including diversity reception, modulation schemes, signal coding and pilot tone systems. Spatial diversity reception involves the use of multiple receiving antennas spaced a distance apart so that signals are received over more than one signal path. As signals from diverse paths typically exhibit uncorrelated fading, they may be combined in the receiver to ameliorate the effects of fading. Similarly, radiotelephone communications signals may be transmitted and received using diverse polarizations and combined at the receiver to take advantage of the low correlation of fading between signals of differing polarizations. Despite the theoretical advantages of these types of diversity reception, however, sufficient diversity gain may not be possible because of limitations on antenna placement.
The modulation/demodulation scheme implemented in the transmitter 54 and receiver 55 can also affect the performance of the radiotelephone communications channel 520. Some modulation schemes provide better performance in a fading environment, but conventional modulation techniques generally provide poor performance without the use of coding. Several coding schemes have been proposed to reduce the effects of fading, but these coding schemes tend to require coherent detection which may be difficult in applications where there are high levels of interference. In addition, these coding techniques may require complex signal processing to recover the radiotelephone communications, creating processing delays which may be unacceptable in certain applications.
Another technique for compensating for the effects of fading in a radiotelephone is to transmit a known pilot signal or tone over at a certain frequency as part of the radiotelephone communications signal. The pilot tone is received by the receiver 55 and used to determine the transfer characteristic of the radiotelephone communications channel 520. Knowing the channel transfer characteristic, the receiver may compensate for the distortion induced by the channel during the process of estimating the symbols being transmitted over the channel.
However, a portion of the transmit spectrum typically must be allocated to the pilot tone, a task which may be problematic. If the tone is placed at a frequency band edge, it can suffer distortion and interference from adjacent frequency bands. If the tone is placed in the center of the frequency band, it may limit the choice of modulation schemes to those schemes which provide for a spectral notch around the carrier band center frequency.
As illustrated in FIG. 6, an alternative to using a pilot tone is to use its time domain analog, i.e., a sequence of pilot symbols which is interleaved with information symbols which carry voice, data and other information, a technique which is often referred to as pilot symbol assisted modulation (PSAM). An encoder 610 typically encodes the radiotelephone communications 505 to produce a sequence of information symbols 615. A multiplexer 620 interleaves this sequence with a sequence of predetermined pilot symbols 602 to produce an interleaved PSAM communications symbol sequence 515. The PSAM communications symbol sequence 515 is communicated over the radiotelephone communications channel 520, producing a data sequence 525 at the receiving station. A decimator 630 separates pilot symbol data 632 corresponding to the transmitted pilot symbols from information symbol data 634 corresponding to the transmitted information symbols. The pilot symbol data 634 is passed through an interpolator 650, typically a lowpass filter, which estimates a transfer characteristic of the radiotelephone communications channel 520. The estimated transfer characteristic is then used in a symbol estimator 640 to compensate estimation of the information symbols in response to distortion induced by the radiotelephone communications channel 520. Analyses of pilot symbol assisted modulation are provided in "TCMP--A Modulation and Coding Strategy for Rician Fading Channels", by Moher et al., IEEE Journal on Selected Areas in Communications, vol. 7, No. 9, December 1989, and in "An Analysis of Pilot Symbol Assisted Modulation for Rayleigh Fading Channels", by Cavers, IEEE Transactions on Vehicular Technology, vol. 40, no. 4, November 1991.
Pilot symbol assisted modulation can provide improved immunity to fading, but may have several drawbacks. In general, the error probability of symbol estimates tends to increase for those symbols which are the furthest distance from the pilot symbols in the symbol sequence, contributing to the overall bit error rate for radiotelephone communications communicated over the channel. In order to reduce the bit error rate, pilot symbols may be inserted in the symbol sequence at a smaller intervals to reduce the separation between the pilot symbols and to increase the accuracy of the estimated channel transfer characteristic. As pilot symbols generally have no information content, however, increasing the frequency of pilot symbols in the transmitted symbol sequence can reduce the potential information capacity of the channel, which may in turn reduce the number of channels which can provided in the system and the quality of each channel. Adding pilot symbols may also reduce power efficiency by wasting transmit power in non-informational symbols. In addition, the optimal interpolating filter which is generally needed to recognize the full benefit of PSAM may have hundreds of taps and may present practical difficulties in implementation.